I just use plain white vinegar, the cheap stuff. It's perfectly fine for cleaning and doesn't leave any harmful residues behind. I just get the biggest jug of the cheapest I can find, which right now is Walmart. And any hand held spray bottle that you can adjust the spray on. The vinegar eventually makes the sprayer stop working, so you will periodically need a new spray bottle.
Fly and gnat populations can vary year to year and season to season, depending on climate and environmental conditions. So PDZ might work during some times and not be effective during peak population times of year. If you have loads of flies or gnats, they are going to find fresh droppings, the more you have of droppings, the more maggots you will end up with. Since the birds only really roost at night, if you scrape your poop boards in the morning, then they won't have anything there to attract flies all day. Out in my run, the covered part is deep litter, flies have never had a problem, and my open fenced run the sun takes care of it, again, not a lot of flies there. Also, if you have too many birds in too small of a run area, you are going to have more droppings building up. Another reason that more space is a good idea, then the droppings are not all accumulating in a small spot.
For gnats, we have some here that just annoy the daylights out of my birds all summer. The two best things I've found work for me are 1. planting rosemary bushes in the run. The birds lay in the shade under them and the fragrant oils in the plants seem to repel the gnats. I do have to put down a layer of hardware cloth around the bases of the plants when I plant them to keep the birds from digging them up, other than that they don't bother them.
2. I have a box fan in one of my coop windows that keeps the air moving over the roosts. Makes it much harder for small flying insects, particularly gnats and mosquito's, to land on my roosting birds.